Thermotectonic evolution of the Eastern Segment of southwestern Sweden: tectonic constraints from U-Pb geochronology

Abstract

The Eastern Segment forms a complex crustal terrane that occupies the southeastern part of the Southwest Scandinavian Domain adjacent to the less deformed 1.65-1.85 Ga Transscandinavian Igneous Belt (TIB). New U-Pb data from the Eastern Segment indicate that orthogneisses and granitoid rocks have protolith ages equivalent to the later magmatic phases of the TIB and that the earliest thermotectonic episodes in the Eastern Segment occurred between 1.70 and 1.61 Ga during the Gothian Orogeny. The comparable ages for Eastern Segment gneisses and TIB rocks represent permissive evidence for the hypothesis that the orthogneiss protoliths intruded into the western margin of Fennoscandia and are not exotic with respect to the pre-Gothian craton.The eastern limit of Gothian deformation is thus interpreted to be an intracratonic deformation front that coincides with the later Sveconorwegian deformation front along the Protogine Zone south of Lake Vanern. The post-Gothian anorogenic period was interrupted, in at least the eastern part of the Eastern Segment, by a thermal and magmatic event at 1.47 Ga that generated granitic dykes and (re)crystallized titanite. Sveconorwegian U-Pb ages and field observations are compatible with a thrusting and exhumation of western supracrustal terranes over the Eastern Segment during the Sveconorwegian Orogeny, followed by isostatic unroofing and cooling of the lower deck at 950-930 Ma. Available, albeit limited, Sveconorwegian titanite ages in the Eastern Segment young from east to west, ranging from 950 to 930 Ma, and are substantially younger than those dated in the western terranes of SSD.

Hallandian 1.45Ga high-temperature metamorphism in Baltica: P–T evolution and SIMS U–Pb zircon ages of aluminous gneisses, SW Sweden

“…The common textural and isotopic characteristics of the protolith zircon suggest a common igneous, almost exclusively 1.70 Ga old, source. 1.70 Ga old orthogneiss protolith ages have previously been demonstrated at the Romeleåsen horst, (red aplitic gneiss at Stenberget, Johansson et al, 1993) and dominates among orthogneisses in the Eastern Segment of the Sveconorwegian province (Connelly et al, 1996;Christoffel et al, 1999;Söderlund et al, 1999Söderlund et al, , 2002Andersson et al, 2002;Möller et al, 2007;Brander et al, 2012;Petersson et al, 2013). These rocks are reworked equivalents to the younger 1.74-1.66 Ga magmatic rocks of the Transscandinavian Igneous Belt (Figs.…”

“…from a "Gothian" 1.75-1.55 Ga metamorphic domain locally overprinted by Sveconorwegian shear zones (cp. Connelly et al, 1996, and references therein) to a Sveconorwegian high-grade metamorphic domain with an older, cryptic, Hallandian metamorphic history at 1.47-1.38 Ga (Johansson et al, 2001;Andersson et al, 2002;Söderlund et al, 2002). Despite the advancements made possible by spot dating techniques, challenges remain in many cases to provide a firm petrologic link between age and metamorphic or magmatic stage, rendering the meaning of the age enigmatic.…”

Growth-related 1.85-1.55 Ga magmatism in the Baltic Shield; a review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events - A reply

“…Åhäll et al 1997;Möller & Söderlund 1997). Given that U-Pb studies using discordant dykes as structural markers have managed to document gneissic, pre-1.40 Ga structures in two different areas (Connelly et al 1996;Christoffel et al 1999), we welcome the opportunity to inform GFF readers about an evolving consensus concerning the recognition of pre-1.40 Ga gneisses in this complex segment characterised by 1.70-1.66 Ga rocks and high-grade Hallandian and Sveconorwegian reworking (Christoffel et al 1999;Söderlund 1999;Andersson 2001).…”

“…Hubbard 1975;Gorbatschev & Welin 1980;Samuelsson et al 1988;Larsson et al 1986Larsson et al , 1990 have stood the test of modern U-Pb geochronology (e.g. Connelly et al 1996;Christoffel et al 1999;Söderlund 1999;Andersson 2001). Second, it unveils an event of dynamic deformation that was either Hallandian in age (c. 1.…”

“…Their interpretation, postulating Gothian metamorphism at 1.65 Ga, has later been questioned (Söderlund 1999;Andersson 2001), partly because the age result (1654±9 Ma) overlaps that of the hosting gneiss (1664±7 Ma) and is unsupported by low Th/U ratios. Also previous U-Pb based inferences of Gothian deformation (Connelly et al 1996) have been questioned (e.g. Söderlund 1999;Andersson 2001).…”